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1. Field of the Invention
The present invention relates to the field of optics and in particular to beam splitting devices, based on nonlinear effects.
2. Background of the Invention
It is known that a laser beam, while typically monochromatic, can include two or more than two frequencies or frequency peaks as a result of frequency doubling or three-wave mixing in a nonlinear medium having a second order nonlinearity. In such a case, it is often desirable to separate the beam into two or more components with different frequencies. Conventionally, an optical filter can be employed to pass one wavelength and reflect the other wavelength(s). Alternatively, dispersive prisms can be used to separate a laser beam containing two or more frequencies using the fact that various wavelengths of light undergo refraction at different angles. However, the first approach entails loss of light energy, and dispersive prisms are characterized by a large angle of refraction and a long length is required to resolve the two beams, which is inconvenient in practice. The present invention aims at alleviating at least some disadvantages of the prior designs.
The above-mentioned frequency changes in light frequency in the nonlinear medium can be explained in simple terms as follows:
A laser beam incident on a nonlinear medium will produce another beam or beams with new frequencies output from the medium due to the nonlinear effect, which is known as frequency doubling or three-wave mixing. In this process, two photons with frequency xcfx891 and xcfx892 are annihilated to create simultaneously a third single photon of frequency xcfx893. This process is known as sum frequency generation. Conversely, one can talk about generating lower frequencies using the three-wave mixing process. The present application proposes a beam splitting device based on the research of the above nonlinear phenomena.
Frequency doubling is a phase-matched process that requires the momentum of the photons to be conserved. This is conventionally obtained by using a birefringent crystal having a second order nonlinearity and propagating the waves through the crystal as e-rays (extraordinary rays) or o-rays (ordinary rays). The directions for the e-rays and o-rays are advantageously chosen or the refractive indices of the o-ray and e-ray are temperature or pressure tuned so as to obtain the phase matching condition. There are three possible ways for combining the fundamental light waves, that is:
o(xcfx89)+o(xcfx89), e(xcfx89)+e(xcfx89), o(xcfx89)+e(xcfx89),
wherein xe2x80x9coxe2x80x9d denotes the ordinary ray and xe2x80x9cexe2x80x9d denotes the extraordinary ray.
Since polarized wave exists also at the doubled frequency, the three above combinations for the light waves at the fundamental frequency may likely result in the following possible combinations, respectively:
o(xcfx89)+o(xcfx89)xe2x86x92o(2xcfx89), o(xcfx89)+o(xcfx89)xe2x86x92e(2xcfx89)
e(xcfx89)+e(xcfx89)xe2x86x92o(2xcfx89), e(xcfx89)+e(xcfx89)xe2x86x92e(2xcfx89)
e(xcfx89)+o(xcfx89)xe2x86x92o(2xcfx89), e(xcfx89)+o(xcfx89)xe2x86x92e(2xcfx89)
However, those combinations where all the waves are either all o-rays or all e-rays are not useful since the phase-matching condition is not possible in birefringent crystals for these cases. Therefore, there are only four possible ways for frequency doubling, which can be classified according to their features as phase-matching of the 1st kind (I) and the phase-matching of the 2nd (II) kind. The I kind, since the polarized directions of the electrical vector of the interacting fundamental light waves are parallel, is also termed xe2x80x9cparallel phase matchingxe2x80x9d, while the II kkind is also termed xe2x80x9corthogonal phase matchingxe2x80x9d since the polarized directions of the electrical vector of the interacting fundamental light waves are mutually orthogonal.
In the process of frequency multiplying, various known optical techniques are used to separate the fundamental frequency light from the doubled frequency for the above-mentioned two kinds of phase-matching. The present invention, defined and described below, proposes a new and different approach.
As for the three-wave parametric processes, the device of the present invention can be used to obtain parallel separation of the light at the single frequency from the light at the idler frequency. As above, there are two possible types for phase-matching for these optical parametric processes. In this case the relationship between the waves at the pump light frequency, signal light frequency and idler light frequency are expressed as follows:             the      ⁢              xe2x80x83            ⁢      I      ⁢              xe2x80x83            ⁢      kind      ⁢              xe2x80x83            ⁢      phase      ⁢              -            ⁢      matching      ⁢              xe2x80x83            ⁢      e        →                  o        +                  o          ⁢                      xe2x80x83                    ⁢          o                    →              e        +        e                                the        ⁢                  xe2x80x83                ⁢        I        ⁢                  xe2x80x83                ⁢        I        ⁢                  xe2x80x83                ⁢        kind        ⁢                  xe2x80x83                ⁢        phase        ⁢                  -                ⁢        matching        ⁢                  xe2x80x83                ⁢        e            →                        o          +                      e            ⁢                          xe2x80x83                        ⁢            o                          →                  o          +          e                      ⁢          xe2x80x83      
In summary, what is disclosed is an apparatus and methods for separating into two parallel beams, the created light frequency from the light at the fundamental frequency or the light at the idler frequency for either the process of frequency doubling or the parametric process of three-wave mixing using either I or II kind phase-matching.
According to the invention, there is provided a beam splitting device, useful for laser beam splitting, the device comprising:
a wavelength-selective polarization rotating means disposed in the path of a light beam having two or more wavelengths for imparting a different polarization direction to different wavelengths of the light beam thus forming two or more sub-beams of light, and
means for spatially separating the sub-beams of light according to their polarization.
In one embodiment, the wavelength-selective means may be a half-waveplate.
In another embodiment, the wavelength-selective means may be an interleaver.
In one embodiment, the means for spatially separating comprise a walk-off crystal disposed to angularly separate the differently polarized sub-beams of light. In another embodiment, the spatially separating means may be a Wollaston prism.
In one embodiment, the device also comprises an optical element, or a plurality of optical elements, disposed in the path of the angularly separated sub-beams of light for directing the sub-beams on mutually parallel paths. The element may be a Wollaston prism, a beveled prism or an equivalent element.
In an embodiment of the invention, the means for spatially separating and the optical element for directing the sub-beams on parallel paths may be a walk-off crystal.
Turning back to the process of light parameter magnifying, the device of the invention can be utilized to realize parallel separation of a signal light beam and idler frequency light. The relationship of pump light, signal light and idler frequency light, encountered for instance in OPO (optical parametric oscillator) can be expressed as follows:             xe2x80x83        ⁢                  phase        ⁢                  -                ⁢        matching            ,                                    1            st                    ⁢                      xe2x80x83                    ⁢          kind          ⁢                      xe2x80x83                    ⁢          e                →                              o            +                          o              ⁢                              xe2x80x83                            ⁢              o                                →                      e            +            e                                          xe2x80x83        ⁢                  phase        ⁢                  -                ⁢        matching            ,                                    2            nd                    ⁢                      xe2x80x83                    ⁢          kind          ⁢                      xe2x80x83                    ⁢          e                →                              o            +                          e              ⁢                              xe2x80x83                            ⁢              o                                →                      o            +            e                              
In other words, it is possible to find suitable ways to resolve the signal light and fundamental frequency light or idler frequency lights into parallel beams either for the process of frequency multiplying and light parameter magnifying or for the 1st and 2nd type of phase matching.